A two-component development system is provided in an image forming apparatus such as an electrophotographic copier or a printer so as to develop, with the use of a developer made up mainly of toner and magnetic particles (carrier), an electrostatic latent image formed on an image carrier. The two-component development system is widely used for a large printing machine capable of high-speed printing.
FIG. 5 shows a typical two-component developing device. According to the developing device in FIG. 5, a developer stirring chamber 80 provided in a developer tank 70 contains a two-component developer made up of nonmagnetic toner and a carrier. The nonmagnetic toner is made up mainly of a polyester resin, and the carrier is in the form of magnetic particles.
Provided in the developer stirring chamber 80 containing the aforementioned developer are: a developing sleeve 91, which is provided adjacent to a photoreceptor drum 94; a stirring screw 81, which is located directly below a toner hopper 60; and the like.
The developing sleeve 91 contains a magnet 92 made up of a plurality of magnetic poles. The developing sleeve 91 (i) regulates, with the use of a blade 93 provided near the developing sleeve 91, an amount of two-component developer to be conveyed, (ii) carries the developer layered on a surface thereof, and (iii) conveys the developer to a developing area of the developing sleeve 91 facing the photoreceptor drum 94. In the developing area, only the toner of the developer is attracted by an electric field generated by electrostatic charges on a surface of the photoreceptor drum 94, and is transferred from the developer.
Rotation of the stirring screw 81 by a rolling mechanism (not shown) (i) stirs the developer entirely in an axial direction and (ii) causes the toner to carry a desired amount of charge while causing the toner and the carrier together constituting the developer to rub against each other. When a toner density sensor 82 detects a decrease in toner density in the developer stirring chamber 80, an operation is carried out such that the toner is dropped from a toner feed opening 61 provided at a lower end of the toner hopper 60. The dropped toner is stirred into and mixed with a residual developer by the rotation of the stirring screw 81.
The toner density in the developer stirring chamber 80 is always monitored by the toner density sensor 82. When the toner density reaches a specified value, the supply of the toner from the toner feed opening 61 is stopped.
In this way, the developer stirred by the stirring screw 81 maintains a constant toner density, and the toner is caused to carry a certain amount of charge.
A developing process carried out at a higher speed requires stirring the toner and the carrier together constituting the developer in a shorter period of time. For this reason, the stirring and mixing of the developer and application of the charge in a desired amount to the toner must be carried out more quickly than in a normal process. Therefore, some sort of effort needs to be made so that the developing device can be used without problems in a high-speed developing process.
According to an electrophotography-use developer disclosed in Japanese Unexamined Patent Publication No. 102865/1992 (Tokukaihei 4-102865; published on Apr. 3, 1992), microparticles of an inorganic oxide having a specific volume resistance are added to toner and a carrier, so that (i) charge exchangeability between the toner and the carrier and (ii) a charging speed are improved. As a result, uncharged toner additionally supplied from a toner hopper into the developer is successfully inhibited from (a) becoming weakly charged and (b) becoming inversely charged, although such uncharged toner is likely to (a′) become weakly charged and (b′) become inversely charged. Addition of the additive to the developer instead of altering a developing process carried out in a developer tank makes it possible to be compliant with a high-speed developing process.
According to a developing device disclosed in Japanese Unexamined Patent Publication No. 333700/1993 (Tokukaihei 5-333700; published on Dec. 17, 1993), attention is paid to a toner density of toner contained in a developer, which toner density is strongly correlated with a saturated charging amount of the toner. A technique of highly accurately controlling the toner density realizes stabilization of the charging amount of the toner.
Concerning the relationship between (i) the saturated charging amount of the toner and (ii) the toner density of the toner contained in the developer, the Kondo theory advocated by Professor Kondo of the Nippon Institute of Technology is widely known. The Kondo theory corresponds to an experimental result with high reproducibility. (See Kondo, A. A Mechanism for Frictional Charging Using Powder Toner, “Society for Electrophotography 43rd Research Symposium”, pp. 26-30 (1979))
(i) a specific charge amount of toner (value of the amount of an electric charge q relative to the mass of toner m, q/m[C/g]) and (ii) a toner covering rate θ (*2) calculated from the number of toner particles (*1) sticking to one carrier particle has a relationship expressed by the following formula:q/m=(aθ+b)−1 
*1 Number of Toner Particles Sticking to One Carrier Particle=(Number of Toner Particles Contained in Developer)/(Number of Carrier Particles Contained in Developer)
*2 Toner Covering Rate θ=(Number of Toner Particles Sticking to One Carrier Particle)×(Projected Area of One Toner Particle)/(Surface Area of One Carrier Particle)
Both *1 and *2 assume that each of the particles is a spherical particle.
Note that the constants a and b are determined depending on physical properties of the toner and the carrier, respectively. The specific charge amount of toner is inversely proportional to the toner covering rate θ, and is a value uniquely decided by the toner covering rate, i.e., by the toner density of the toner contained in the developer. Controlling the charging amount of toner to be stable is equivalent to controlling the toner covering rate to be constant, and is nothing but managing the mass of toner relative to the mass of the developer.
According to a developing device disclosed in Japanese Unexamined Patent Publication No. 172879/1989 (Tokukaihei 1-172879; published on Jul. 7, 1989), a toner replenishing roller for controlling an amount of toner to be supplied from a toner hopper is provided with a frictional charging function, so that toner charging rise-up properties obtained when the toner is supplied is improved. According to the toner replenishing roller provided with the charging function, the toner to be supplied is sandwiched between the toner replenishing roller and a metal or resin blade so as to be rubbed, and is given an electrical charge in advance. Since the toner supplied to a developer is always charged toner, no defect occurs in a developed image even if the supplied toner reaches a developing area without having been stirred into a carrier in a stirring chamber.
As described above, a developer of a two-component developing method has an arrangement such that toner and a carrier are quickly stirred and the toner carries electric charge in a desired amount. This realizes (i) improvement in toner charging rise-up performance and (ii) stabilization of the amount of an electric charge.
However, also in each of the aforementioned conventional examples, an arrangement of a developer tank designed for high-speed development has a problem described below.
As described above, according to the developing device disclosed in the aforementioned Japanese Unexamined Patent Publication (Tokukaihei 5-333700), the stabilization of the charging amount of the toner is realized by highly accurately controlling the toner density of the toner contained in the developer. Here, attention must be paid to the fact that this function is effective only in cases where the saturated charging amount of the toner is evaluated. Generally, the saturation of the charging amount of toner requires charging time that depends on the physical property of the toner. There is no problem under a process condition where a sufficient amount of charging time is secured. However, in cases where the process is carried out at a higher speed and it is therefore impossible to secure sufficient stirring time for the developing device or the like, the toner falling short of the saturated charging amount reaches a developing area, and taints a developed image.
Further, according to the developing device disclosed in the above-mentioned Japanese Unexamined Patent Publication (Tokukaihei 1-172879), the toner is rubbed between the toner replenishing roller and the metal or resin blade. In such a developing device, the problem of toner fusion with a blade cannot be avoided, as with the case of a toner layer forming blade using a one-component developing method. The toner is put under great stress when the toner is rubbed, and it impossible to guarantee long-term stability of toner charging.
When the blade is tainted due to the toner fusion, the toner to be supplied cannot be given a sufficient amount of electric charge, so that uncharged toner remains in the developer. This causes toner fogging or smudges on a sheet of paper. The problem of toner fusion appears more remarkably in case where toner designed for a high-speed developing process and made up of a soft material slightly shifted toward a low molecular weight is used. Therefore, it is necessary to take measures against the problem.
A high-end ultrahigh-speed copier (model having a copying speed of 70 or more copies per minute in monochrome copying and a copying speed of 50 or more copies per minute in color copying) recently launched on the market by each company is approaching the limit of toner charging rise-up performance. Since the stirring performance of a developer is determined depending upon respective diameters of a stirring screw and a developing sleeve, it is possible to satisfy a desired property by enlarging those components. However, since there have been strong consumer needs for a small copier, there is a limit on the maximum size of the components.
For example, assume a developing process (i) in which a developing device which outputs 70 sheets of A4-size paper per minutes in a landscape orientation is used, (ii) in which a developing sleeve has a diameter of 30 mm, and (iii) in which the developing sleeve rotates so that a surface of the developing sleeve moves 1.5 times faster than a surface of a photoreceptor body carrying an electrostatic latent image. In such a developing process, the surface of the developing sleeve has a circumferential velocity of 36.6 cm/sec, and the rotational velocity reaches 233 rpm. The increase in the rotational velocity of the developing sleeve causes a stirring screw to rotate at a higher velocity. The increase in the rotational velocity of the stirring screw causes the toner to stay in a stirring chamber for a shorter period of time. As a result, it becomes inevitable that a developer is stirred in a shorter period of time. In cases where such a stirring mechanism as described above is adopted, toner charging failure, particularly a high proportion of uncharged toner, is likely to occur under such conditions that the circumferential velocity of the surface of the developing sleeve exceeds approximately 36 cm/sec.
It is an object of the present invention to provide a developing device (i) which solves the foregoing problems caused when a two-component developer is used in a high-speed developing process, (ii) which is capable of causing uncharged toner to carry a desired amount of charge, by quickly mixing the uncharged toner with a carrier and stirring the mixture even when the uncharged toner is continuously supplied from a toner hopper, (iii) which realizes (a) further improvement in toner charging rise-up performance and (b) further stabilization of the amount of electric charge for charging, and (iv) which is always capable of developing a high-quality image.